As is known in the art, a portable hand-held telephone can be arranged in an automobile or other vehicle so that a driver or other occupant of the vehicle can place and receive telephone calls from within the vehicle. Some portable telephone systems allow the driver of the automobile to have a telephone conversation without holding the portable telephone. Such systems are generally referred to as “hands-free” systems.
As is known, the hands-free system receives acoustic signals from various undesirable noise sources, which tend to degrade the intelligibility of a telephone call. The various noise sources can vary with time. For example, background wind, road, and mechanical noises in the interior of an automobile can change depending upon whether a window of an automobile is open or closed.
Furthermore, the various noise sources can be different in magnitude, spectral content, and direction for different types of automobiles, because different automobiles have different acoustic characteristics, including, but not limited to, different interior volumes, different surfaces, and different wind, road, and mechanical noise sources.
It will be appreciated that an acoustic source such as a voice, for example, reflects around the interior of the automobile, becoming an acoustic source having multi-path acoustic propagation. In so reflecting, the direction from which the acoustic source emanates can appear to change in direction from time to time and can even appear to come from more than one direction at the same time. A voice undergoing multi-path acoustic propagation is generally less intelligible than a voice having no multi-path acoustic propagation.
In order to reduce the effect of multi-path acoustic propagation as well as the effect of the various noise sources, some conventional hands-free systems are configured to place the speaker in proximity to the ear of the driver and the microphone in proximity to the mouth of the driver. These hands-free systems reduce the effect of the multi-path acoustic propagation and the effect of the various noise sources by reducing the distance of the driver's mouth to the microphone and the distance of the speaker to the driver's ear. Therefore, the signal to noise ratios and corresponding intelligibility of the telephone call are improved. However, such hands-free systems require the use of an apparatus worn on the head of the user.
Other hands-free systems place both the microphone and the speaker remotely from the driver, for example, on a dashboard of the automobile. This type of hands-free system has the advantage that it does not require an apparatus to be worn by the driver. However, such a hands-free system is fully susceptible to the effect of the multi-path acoustic propagation and also the effects of the various noise sources described above. This type of system, therefore, still has the problem of reduced intelligibility.
A plurality of microphones can be used in combination with some classical processing techniques to improve communication intelligibility in some applications. For example, the plurality of microphones can be coupled to a time-delay beam former arrangement that provides an acoustic receive beam pointing toward the driver.
However, it will be recognized that a time-delay beamformer provides desired acoustic receive beams only when associated with an acoustic source that generates planar sound waves. In general, only an acoustic source that is relatively far from the microphones generates acoustic energy that arrives at the microphones as a plane wave. Such is not the case for a hands-free system used in the interior of an automobile or in other relatively small areas.
Furthermore, multi-path acoustic propagation, such as that described above in the interior of an automobile, can provide acoustic energy arriving at the microphones from more than one direction. Therefore, in the presence of a multi-path acoustic propagation, there is no single pointing direction for the receive acoustic beam.
Also, the time-delay beamformer provides most signal to noise ratio improvement for noise that is incoherent between the microphones, for example, ambient noise in a room. In contrast, the dominant noise sources within an automobile are often directional and coherent.
Therefore, due to the non-planar sound waves that propagate in the interior of the automobile, the multi-path acoustic propagation, and also due to coherency of noise received by more than one microphone, the time-delay beamformer arrangement is not well suited to improve operation of a hands-free telephone system in an automobile. Other conventional techniques for processing the microphone signals have similar deficiencies.
It would, therefore, be desirable to provide a hands-free system configured for operation in a relatively small enclosure such as an automobile. It would be further desirable to provide a hands-free system that provides a high degree of intelligibility in the presence of the variety of noise sources in an automobile. It would be still further desirable to provide a hands-free system that does not require the user to wear any portion of the system.